Megakaryocytes from bone marrow of mice and guinea pigs will be obtained so that the cell biology of these platelet progenitors could be studied in vivo and in vitro. Megakaryocyte maturation, cytoplasmic granule formation, and platelet formation will be studied by: light, phase, phase interference microscopy, cinemicrophotography, electron microscopy, fluorescence microscopy, and autoradiographic techniques. Megakaryocytes will be isolated from bone marrow employing techniques of velocity sedimentation, and optimal conditions for maintaining them in tissue culture will be determined. Studies will be directed toward: formation and identification of possible types of granules in megakaryocytes; function of microtubules and microfilaments in maturation and platelet formation; response of these organelles to various chemical compounds. Enlargement of the megakaryocyte pool acutely (by bleeding) or chronically by specific hormones (i.e., thrombopoietin) will be employed to increase the yield of these cells.